1. Field of the Invention
This invention relates to core drill bits and more particularly, to a core drill bit which includes multiple drilling fluid jets circumferentially spaced on the contact end, face or crown of the bit and an annular screen mounted in the drill bit body adjacent to the mouth of the jets. As the bit cuts into a subterranean interval or formation, the core of the formation is forced upwardly through the center of the bit into the core barrel attached to the bit. Drilling fluid is also pumped from a mud tank on the ground surface through the drill string and core barrel and into the bit, through an annulus surrounding the core of the formation. As it is pumped from the core barrel and bit into the drilling fluid jets, the drilling fluid passes through the screen into an annular equilization channel, which screen blocks oversized formation and circulating "mud" fragments, gravel and particulate matter large enough to plug the jet nozzles, The screened drilling fluid then flows through the jets and jet nozzles, impinges on the formation and is pumped around the core barrel and drill string up to the ground level, where it is filtered in the mud tank and redirected into the drill string and core drill bit.
2. Description of the Prior Art
Conventional drill bits are typically characterized by multiple cutting blades or elements circumferentially mounted on the contact end, face or crown of the drill bit. Drilling fluid jet nozzle are typically interposed between the cutting elements to aid the interval cutting action. The cylindrical drill bit body is provided with interior threads which are engaged by threads provided on the exterior circumference of a hollow drill pipe and as the drill pipe is rotated with the drill string, the drill bit cuts through the down-hole interval or formation. In a typical drilling application, drilling "mud" or fluid is pumped from a mud tank on the ground surface, down the drill string bore and into the drill bit and is ejected from the jet nozzles, thus assisting the cutting elements in cutting through the formation. The drilling fluid carries the dislodged formation particles to the surface, where the drilling fluid is filtered in the mud tank and the cycle repeats. In a second drilling application, a core drill bit having a hollow core is used to collect core samples. As the core drill bit cuts into the formation, a cylindrical core of dislodged formation material is pushed upwardly through the core of the drill bit and into the connecting core barrel. Drilling fluid is pumped from the mud tank down through the drill string and core barrel, into the drill bit through an annulus which surrounds the isolated formation core. When the drilling fluid reaches the drill bit, it enters several circumferentially-spaced drilling fluid jets provided on the end or crown of the drill bit and is ejected at high pressure from the jet nozzles terminating the end of the drilling fluid jet. The drilling fluid assists the cutting elements in dislodging formation material and carries loose formation particles and formation cuttings to the mud tank or pit on the surface. At the proper time, the core barrel and core drill bit are removed from the well with the drill string and the formation material collected in the core barrel is removed for analysis.
One of the problems encountered in using conventional core drill bits for obtaining formation samples as described above, is realized when the drilling fluid is pumped down the annulus under high pressure and carries or entrains formation particles or gravel of various size from the core of dislodged formation material collected in the core barrel. The drilling fluid carries these particles into the drilling fluid jets and some of the particles are large enough to block the jet nozzles so that drilling fluid cannot be ejected through the nozzles at peak efficiency. This event greatly reduces the drilling efficiency of the core bit and the drilling operation. Bag-type screens are typically installed in the drill string to reduce the incidence of jet nozzle plugging by screening the circulating well fluid, but these devices are inoperable to screen particles entrained in the well fluid from the formation core.
The core barrel drill bit of this invention includes an annular screen mounted in the drill bit body, and drilling fluid which is pumped down the annulus in the core barrel passes through the screen openings before entering the drilling fluid jets. The screen openings are sufficiently small that drilling fluid and formation particles dislodged from the core barrel and large enough to block the jet nozzles are prevented from entering the drilling fluid jets and thus cannot block the jet nozzles.
Accordingly, it is an object of this invention to provide a new and improved core drill bit which includes screened drilling fluid jet nozzles.
Another object of this invention is to provide a core drill bit which includes a woven wire or perforated plate screen mounted in the body of the bit to prevent formation gravel and other particles in the circulating fluid from entering and blocking the drilling fluid jet outlets in the drilling face or crown of the bit.
Still another object of the invention is to provide a new and improved core barrel drill bit which is characterized by multiple cutting elements circumferentially spaced on the drilling face or crown of the bit, multiple drilling fluid jets interposed between or in the cutting elements and an annular equilization channel fitted with a screen mounted in the body of the bit adjacent to the jets, to prevent formation, circulating "mud" and other particles from entering and blocking the jet nozzle orifices of the drilling fluid jets.